Battery separators are distinguished in the art as primary battery separators and secondary battery separators. A secondary battery separator, such as a separator for a conventional lead/acid battery, requires very different properties as opposed to a separator for a primary battery, e.g., an alkaline cell battery. Secondary batteries, normally, are rechargeable many times, while primary batteries, if rechargeable at all, are rechargeable to a very limited degree. As a result, the materials required for a secondary battery separator are substantially different from the materials required for a primary battery separator.
A number of materials have been used in the prior art in connection with secondary batteries, but the acceptability of those materials for primary batteries cannot be predicted from acceptability in a secondary battery, and, most often, separators useful in a secondary battery are not useful in a primary battery. While a wide range of separators have been successfully used in secondary batteries, e.g., plastics, wood, wood pulp, rubber, and the like, materials which have been found acceptable for primary batteries are far more limited. This is particularly true in regard to alkaline primary batteries, such as a conventional alkaline cell battery, since the mode of manufacture thereof is considerably different from the mode of manufacture of a conventional secondary battery, e.g., a lead/acid automobile battery, and an alkaline cell primary battery separator must be capable of operating in a highly alkaline medium, as opposed to a low pH acid medium.
Thus, acceptable separators for primary batteries, and especially alkaline cell batteries, have been substantially limited in the art. However, the preferred alkaline cell battery separator is made of polyvinyl alcohol fibers. These fibers are, essentially, unique in this art, in that they can be formed into a flexible mat of thin cross-sections to allow usual manufacture of the cells, but at the same times these fibers are stable, particularly dimensionally stable, at the high alkaline pHs of an alkaline cell battery. Accordingly, most modern alkaline cell batteries use battery separators made of polyvinyl alcohol fibers.
In order to form a mat of the polyvinyl alcohol fibers, dissolvable or partially dissolvable polyvinyl alcohol fibers are mixed with non-dissolvable polyvinyl alcohol fibers in a convenient solvent, usually water. After sufficient dissolution of the dissolvable polyvinyl alcohol fibers, the mixture is then formed into a shape-sustaining form, e.g., a mat, and dried. The dissolved polyvinyl alcohol forms a matrix about the non-dissolvable polyvinyl alcohol fibers and thus keeps that mat in a shape-sustaining form. In addition, that matrix forms a permeable barrier between the polyvinyl alcohol fibers for appropriate ionic transfer during discharge of the alkaline cell battery.
While such battery separators are the preferred form in the art, they do suffer from several disadvantages. Firstly, due to variables in manufacture, especially the above-noted dissolving step, the strength of the battery separator itself may vary considerably. The matrix formed by the dissolved polyvinyl alcohol is not a particularly strong matrix, and the polyvinyl alcohol fibers, themselves, are not particularly strong fibers in the wet state. Thus, in the manufacture of the battery separators, differences in the non-dissolved fibers and in the dissolved matrix can result in a formed mat that is subject to tearing. Further, the polyvinyl alcohol matrix, while providing a permeable matrix, tends to produce considerable variation of permeability, which results in uneven ionic transport across the battery separator, and somewhat variable electrical discharge thereof. Also, the process of manufacturing both the non-dissolvable polyvinyl alcohol fibers and the dissolvable polyvinyl alcohol fibers inherently produces variabilities in these fibers. This results in variability in the matrix formed by the dissolved fibers and in the permeability property, of the non-dissolvable fibers/matrix forming the battery separator.
However, probably of more importance than any of the foregoing disadvantages of these conventional alkaline cell battery separators is the penetration of those conventional battery separators by dendrite formation. As is well known, in such alkaline cell batteries, the metallic component of the battery, e.g., lead or zinc, is separated from the other battery components by the battery separator. That battery separator, during manufacture of the battery, is wetted with a highly alkaline solution in order to provide an ionic transport between the two components of the battery. During use of the battery, and even during non-use and during storage, dendritic structures form from the metal component of the battery. If these dendritic structures continue to form and enlarge, they can pierce through the battery separator and contact the other component of the battery, thus, providing a direct short of the battery, and, of course, resulting in a decreased life or unserviceability of the battery. While the polyvinyl alcohol battery separators are resistant to the highly alkaline solution wetted therewith, these conventional separators are relatively easily pierced by such dendritic structures, and the variability in porosity of the separators allows more easy formation of the dendritic structures through the battery separator, causing such shorts in the battery.
In view of the foregoing, the art has long sought methods of improving the relatively unique polyvinyl alcohol battery separator. However, for the reasons explained above, this effort in the art has been difficult and unsuccessful, primarily because of the demanding properties of alkaline cell battery separators. It would, therefore, be of substantial advantage to the art to provide improved polyvinyl alcohol alkaline cell battery separators, which mitigate the disadvantages noted above.